This invention relates generally to electrostatographic copying apparatus and more particularly to a sheet delivery system for such apparatus. In particular it is directed to an improved sheet delivery system for use in apparatus wherein the toner image is at least, in part, fixed to a support sheet by the application of heat.
In the process of xerography a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photoconductive insulating member with subsequent rendering of the latent image visable by the application to the photoconductive insulating member of electroscopic marking particles commonly referred to as toner. This visual image can either be fixed directly on the photoconductive insulating member or transferred from that member to a image receiving member such as plain paper with subsequent affixing of the image thereto.
It is common practice to permanently affix or fuse the toner material onto the support member by heat. In this process it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner become tacky and coalesce. At this time the softening causes the toner to be absorbed to some extent into the fibers of the support member which in many instances is plain paper. Thereafter as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member. Typically the heat necessary to provide such fusing is supplied with a radiant fuser device, such as that generally illustrated in FIG. 1.
In a typical commercial embodiment, such as the Xerox 3400, or 3450 products, to facilitate handling of the individual copies a sheet delivery system is provided adjacent to the radiant fuser. Typically the sheet delivery systems comprise a pair of rolls, one of which is driven, the other which is friction driven by being in contact with the driven roll. Typically these rolls get hot after prolonged use as a result of the proximity to the fuser which is at elevated temperature and the contact with heated paper. This is particularly prevalent in the high use, high volume machines which are constantly run over long periods of time so that heat buildup in the delivery rolls becomes excessive. For example, on long runs, it has been found that the delivery rolls may reach a temperature of up to 180.degree. F., a temperature at which the toner will not solidify but softens and transfers to any surface with which it comes in contact. In the sheet delivery system, such as that illustrated in FIG. 1, the softened toner may offset from the copy sheet to the transport roll with which it comes in contact. And since this roll is hot from prolonged use in proximity to the fuser and being in contact with the heated paper, the toner may remain soft and transfer to subsequent copies. Thus the transport roll may pick up tone from a first sheet of paper and transfer it back onto subsequent copy sheets. In addition, paper fibers or other debris may be held by the soft toner on the transport roll. Both of these effects may be particulary pronounced with a sheet delivery roll which has ridge points which are circular raised rings around the periphery of the transport roll and which serve as the driving force between the idler top transport roll and the bottom driven roll. With the toner offsetting to these ridge parts, offsetting of the toner onto subsequent copies occurs in the form of streaks.
Furthermore, after the toner builds up on the roll to any significant degree, the paper may tend to stick to the roll causing a paper jam. Once the jam is caused in the delivery roll system, it is possible that the paper may buckle within the fuser and come in contact with fuser elements. When this happens the paper overheats, tends to char and eventually contaminates the system. Once the system is contaminated a skilled technician may be needed to clean the entire fuser and sheet delivery system.